In the manufacture of integrated circuits, after individual devices such as transistors have been fabricated on the semiconductor substrate, they must be interconnected in order to perform the desired circuit functions. This interconnection process is generally called “metallization” and is performed using a number of different photolithographic, deposition, and removal techniques.
A commonly used process for forming interconnect structures is referred to as a “damascene” process. In a typical damascene process, dielectric layers are deposited over the devices, followed by the formation of openings in the dielectric layers. Conductive materials are then deposited in the openings. A polish process is used to planarize the conductive materials with the surfaces of the respective dielectric layers so as to cause the conductive materials to be “inlaid” in the respective dielectric layers.
Copper is typically used for the damascene processes. Copper has low resistivity, thus the RC delay caused by the resistance in the interconnect structure is low. However, with the scaling of integrated circuits, the dimensions of copper interconnects are also down-scaled. When the dimensions of the copper interconnects approach the mean free path of electrons, the resistivity of the interconnect structure significantly increases. As a result, the RC delay from the interconnect structure significantly increases.
Various methods have been explored to reduce the resistivities of the interconnect structures. For example, diffusion barrier layers, which are used to prevent copper from diffusing into neighboring low-k dielectric layers, typically have high resistivities. Methods for forming thinner barrier layers are thus used. Also, the scattering of electrons at the boundaries of the copper grains contributes to the increase of the resistivity, and thus various methods have been proposed to increase the copper grain size, hence reducing the scattering of electrons at the grain boundaries.
With the constant reduction in the size of interconnect structures, methods for lowering resistivity are always demanded. Particularly, as the dimensions of interconnect structures become smaller and smaller, phenomena that used to be insignificant begin to contribute more to the overall resistivity. Accordingly, new methods adjusting to these factors are needed.